THE SEA OTTER IN THE EASTERN PACIFIC OCEAN 239 



centage of dependent young in the population is relatively low 

 in the winter and spring. After the late-winter to early-spring 

 season of storms, which is accompanied by much mortality among 

 juvenile otters (see Limiting Factors), an increasing number of 

 births is demonstrated in the field counts. A summer peak in the 

 frequency of births is confirmed by high frequency of large fetuses 

 recovered in that season from reproductive tracts (table 47). 



The frequency of nearly full-term fetuses in the samples in- 

 creased from none in the January-February sample to 6 percent 

 in the March-April sample and to 31 percent in the May-August 

 sample (table 47). The number of dependent young with mothers 

 also increased throughout the spring and summer (see table 52, 

 Field Counts). 



In the small midsummer sample (August) the fetuses were 

 large, in weight classes 3, 4, and 5, none were in weight classes 1 

 and 2 (table 50). 



These data strongly suggest that birth frequency increases in 

 early spring and reaches a peak in summer. Our samples from the 

 September through February period were too small to confirm 

 the field observations of newly born young in every month. 



Although the Steller's sea cow (Hydrodamalis gigas) is a very 

 different animal from the sea otter, it was also a resident inshore 

 marine mammal. It is interesting that Steller noted {in Stejneger, 

 1936, p. 355) that the sea-cow ''brings forth their young at all 

 seasons, generally however in autumn, judging from the many 

 new-born seen at that time." The fact of bearing young at all 

 seasons, while having a season of maximum birth frequency, may 

 be a case of convergent evolution related to the habitat in which 

 these animals were sympatric. 



Place of birth and fetal orientation 



There is no record that the birth of a sea otter has ever been 

 observed. Fisher (1940a, p. 132) presumed that "The births of the 

 young otters take place on the kelp beds." This infers that the 

 young are born in water. Barabash-Nikiforov (1947, p. 97, English 

 translation), however, saw a mother with a newly born young on 

 shore and recovered the still warm afterbirth. 



The place of birth in marine mammals may be related to fetal 

 orientation at birth. For example, in cetaceans birth always occurs 

 in water. Slijper (1956) summarized data on the position of the 

 cetacean fetus at birth. In 22 observed births, all young were born 

 tail foremost. Presumably caudal presentation is a survival factor 



